SAN MARCOS  Growing up in the Twin Oaks Valley, Chris Johnson knew all the best tracks.

There was one by Jack’s Pond, two near the corner of Borden Road and Woodward Street, and several scattered around the hills of the modern-day Twin Oaks Golf Course. These were the dirt tracks of the 1990s where boys would show up with BMX bikes and shovels to shape the ground and test their jumping skills.

Bike jumping was Johnson’s reason to exist, in a sense — until the inevitable accident, a wreck that left him with a broken ankle and a bleak set of options.

Johnson, you see, was born with a rare condition called sacral agenesis, which led to deficiencies of nerves and muscle tissue in his lower legs.

Because of his disability, the doctors said restorative surgery would take a year to heal after his crash. One of them floated the word “amputation” — and explained that a prosthetic lower leg would be much more useful than his natural foot, anyway.

It wasn’t an easy decision for an 18-year-old, Johnson recalled: “When my foot was amputated, I’d get in my wheelchair and wheel down to the jumps. The jumps were my place — even though I didn’t have my bike and I couldn’t jump, I’d just go down there and hang out, watch all the other kids ride their bikes.”

Even after he healed, rid- ing never felt quite the same.

Until recently, that is.

Johnson was interning at Southern California Orthotics and Prosthetics last year when he absent-mindedly hopped across the lab on his prosthetic leg while having the brace on his other foot repaired.

“All of a sudden, a few of the practitioners broke out into a round of applause,” he recalled. “One of them asked if I had ever heard of the Challenged Athletes Foundation, and soon after that I applied.”

The foundation wanted him to train for six months to prove his commitment, which he did. So in addition to coaching and training, it gave him his first professional road bike six months ago. It was resting by his side when we met, having carried Johnson the five miles from his home to the Palomar College campus.

Few can say they learned to ride a bicycle in the same year that they learned to walk, but that is one of Johnson’s achievements. He was 6 — and the doctors had said he wouldn’t even be able to sit up as a baby.

I met him Thursday at Palomar before he headed off to an 11 a.m. session of his last junior college class before he transfers to Cal State San Marcos.

He told me he plans to earn his bachelor’s degree in psychology across the freeway, and then head north, to Cal State Dominguez Hills — home to a renowned prosthetics program.

“The experience I have had with the orthotics and prosthetics, it’s given me opportunities in life that I wouldn’t have had otherwise,” Johnson told me. “There’s nothing I want more than to give that back to other people.”